1. Field of the Invention
The present invention relates to a semiconductor device driving an output node, and specifically, to a semiconductor device with a bus terminating function.
2. Description of the Background Art
In a semiconductor device, a transistor connected to a pin terminal is easy to be affected by noises, since it is directly connected to the outside of the device via the pin terminal. Among the noises, one at a level enough to destruct a device (a transistor) is referred to as a surge. A destruction of a semiconductor device by the surge is referred to as electrostatic damage (ESD), which is accompanied by breakdown of a gate insulating film of an MOS transistor (insulated gate field effect transistor) and others. Therefore, for the reliability of the semiconductor device, the breakdown voltage enough to withstand the surge is required.
For an input pin receiving an external signal, usually, an input protection circuit is configured by a diode or a diode-connected MOS transistor (insulated gate field effect transistor), or a field transistor with a sufficiently thick gate insulating film. By this input protection circuit, the surge is caused to flow to a power supply terminal or a ground terminal, and thus transmission of the surge to the internal circuitry is prevented.
The protection circuit is not required for an output circuit, since an output transistor serves as a surge absorbing transistor. However, in an MOS output circuit configured with an MOS transistor, a large current may flow into the output transistor or high electric field may be generated at drain due to the surge voltage, which may result in electrostatic damage. In order to reduce the current and the drain electric field for avoiding such electrostatic damage, the resistance value of drain region (hereinafter referred to as a drain diffusion resistance) in the output transistor must be increased. Usually, in order to increase the drain resistance, the distance between the gate of the output transistor and a drain contact for connecting to the output node is required to be sufficiently long. Consequently, the diffusion region area of the drain portion in the output transistor increases, and hence, the size of the output transistor increases.
An arrangement for preventing electrostatic damage with limited area of the output circuit is proposed in Japanese Patent Laying-Open No. 2001-127173. According to this first prior art document, drain diffusion region is made different in impurity concentration from source diffusion region, to have an increased drain diffusion resistance value.
In a second prior art document, Japanese Patent Laying-Open No. 11-214621, an arrangement is disclosed in which a terminating resistance element and a protection element for this terminating resistance element are provided between an output transistor and an output pad. According to this second prior art document, the electrostatic protection element is formed of an MOS transistor, and in order to increase the resistance of the drain region, i.e., drain diffusion resistance, the distance between gate and drain contact of the MOS transistor is made longer. Taking advantage of this large drain diffusion region area, the terminating resistance element is arranged above the drain region of the electrostatic protection element in order not to increase the layout area of the entire output circuit. The terminating resistance element is a current limiting resistance element for avoiding a reflecting wave such as a ringing in signal transmission, and is connected between the output pad and the output node (drain) of the output transistor.
In a third prior art document, Japanese Patent Laying-Open No. 10-65744, an arrangement is disclosed in which impedance switching means is provided between an output terminal and an output circuit. The impedance switching means is set to a low impedance state in transmission and to a high impedance state in reception, to reduce reflection noises due to capacitive load of transmission path.
According to the arrangement shown in the first prior art document, the impurity concentrations of source and drain in the output transistor must be made different, and hence, the number of manufacturing steps increases. The drain diffusion resistance is continuously connected to an external bus via the pin terminal. If the drain diffusion resistance functions as terminating resistance, then an output signal is driven via high drain diffusion resistance in signal outputting, and thus the signal can not be transmitted at high speed.
According to the arrangement disclosed in the second prior art document, the protection circuit for the terminating resistance element is arranged corresponding to the output circuit. The distance between a drain contact and a gate electrode of the MOS transistor of the protection circuit is large enough to place the terminating resistor there above. Accordingly, the interface area between the drain region and the substrate region is made large, and a large drain junction capacitance of this protection circuit is connected to the output pin terminal. Consequently, the parasitic capacitance of the output pin terminal increases, and thus, a signal can not be transmitted at high speed. Further, since the terminating resistance element is connected between the drain of the output transistor and the output pad and functions as a current limiting element for an output signal, the signal can not be output at high speed. Moreover, since the terminating resistance causes a voltage drop, a signal at CMOS level can not be transmitted.
According to the arrangement disclosed in the third prior art document, the impedance switching means is provided for the output pin terminal, which has the impedance switched between a transmission mode and a reception mode. In this case also, however, a protection element against the surge must be provided, which increases the occupying area of the circuit for switching the termination resistance value in accordance with the operation mode. Especially, if the circuit portion for controlling this termination resistance is formed with a transistor having an increased drain diffusion resistance, then the load of the output pin increases accordingly. Thus, a signal can not be transmitted at high speed, and the occupying area of the output circuit increases further. In the third prior art document, the consideration is only given to suppress the ringing in signal transmission and reception, and an arrangement for reducing the output circuit area and for alleviating the load of the transmission path is not considered.